2. Field of the Invention
The present invention concerns a process for efficiently producing a molding product of composite material based on Al, Al alloy, Cu or Cu alloy powder while securing required properties, as well as composite material obtained by such a process.
2. Description of the Prior Art
Since Al and Al alloy have the desirable properties of light weight, high-electrical conductivity, thermal conductivity, formability, etc., as well as the ability to be modified with ease in view of the strength by means of alloying, such materials are preferably used in the field of electronic and electric equipment parts, as well as various mechanical parts, for which reduction in the size and the weight is important. Further, in the field of electronic and electric equipment parts, there is an increasing demand for motors of reduced size and weight.
Although steel plate lamination type rotors have generally been used for conventional induction motors, it is difficult to change the speed of such motors with high accuracy solid rotors at high torque and low speed are now being reconsidered for power motors requiring control.
Further, rotors made of die cast aluminum used for high speed rotation (higher than 10,000 rpm) can not withstand centrifugal force. There is a demand for ferromagnetic materials of high electric conductivity for use in such motors.
Furthermore, along with the development for office or factory automation, use of electric equipments having magnetic disc memories have been increased more and more. Thus, magnetic shield structures have become important for possessing records in the magnetic disc memories, as well as it is a legal requirement to prevent electromagnetic wave noises from interferring with other electric equipments. There can be mentioned, as such an example, a precise motor for driving the magnetic head of a magnetic disc memory, for which an electromagnetic shield structure is required and more improvement has been demanded for the performance of the shield.
In a case where magnetic material of higher electric conductivity is required for the electromagnetic wave shield material or rotor material for the induction motor, since performance can not exceed the physical values inherent to Al, Cu and Cu alloy is used instead.
In view of these considerations, magnetic Al or Cu composite material has now been developed and, a method described, for example, in Japanese Patent Application Laid-Open Sho 57-51231 or 61-104040 has been proposed. The magnetic Al composite material described in the former is prepared by uniformly mixing a powder consisting of Al or Al alloy with a ferromagnetic metal powder at a ratio from 20 : 1 to 1 : 1 by weight, pressure molding the mixture and then sintering the compact at a temperature lower than the melting point of Al or Al alloy. The magnetic Al alloy disclosed in the latter application has been proposed by the present inventors and prepared by blending Al or Al alloy with 3 to 60% by weight of a of fiberous ferromagnetic material and then compressing under or after heating at 250.degree. to 650.degree. C. These composite materials have now been noted as new type of magnetic material in which magnetic properties derived from the ferromagnetic metal powder are added to the features of the Al or Al alloy (light weight, workability, electric conductivity, etc.).
However, since the amount of the ferromagnetic material contained in the magnetic Al composite material described in the above-mentioned patent publications is, less than 50% or less than 60% at the maximum as from 20 : 1 to 1 ; 1 by weight or from 3 to 60% by weight, the magnetic flux density under the conditions of the low magnetic field usually employed (about 100 Oe) is extremely small and can not be said to satisfy the required performance for the ferromagnetic material.
By the way, when magnetic properties were measured for the Al composite material solidified in accordance with the method of the above-mentioned prior art by varying the addition amount of ferrous material within a range from 1 to 80% by volume fraction (Vf), a graph as shown in FIG. 5 was obtained. The magnetic property was evaluated based on the magnetic flux density B (gauss (G)) of a specimen disposed under the magnetic field of 100 oersted (Oe). As can be seen from FIG. 5, the magnetic flux density (B) at Vf=25% is 1100 (G) and the magnetic flux density (B) at Vf=34% is 2000 (G). As compared with the magnetic flux density (B) of pure iron at 17600 (G), it is apparent that the value is extremely low in the Al composite material undergoing the restriction in the blending amount of the ferrous material (less than 39%) and it can not be said that the function as the soft magnetic material is sufficiently possessed.
Accordingly, the magnetic Al or Cu alloy utilized at present has not yet been quite satisfactory but leaves room for improvement. That is, the magnetic Al or Cu alloy is prepared by dispersing a ferromagnetic powder such as an iron powder into an Al or Cu powder and then molding them, and there is a need for improving the electric conductivity and the magnetic performance in order to enhance the shielding performance (reflection efficiency, etc.) as the electromagnetic wave shield material, as well as the rotor material used for induction motors.
The conventional method of producing Al or Cu composite material molding products can be classified mainly into the following three methods.
(1) A method of finishing material molded by means of extrusion, hot-pressing, HIP, etc. into a final shape by machining.
(2) A method of cold or hot forging material obtained by extrusion molding and then finishing into a final shape by machining.
(3) A method of compacting a powdery raw material (compact powder), degreasing, applying cold or hot forging and then finishing into a final shape by machining.
Referring at first to the method (1) above, there is a problem that to obtain a part of complicated shape much material must be cut away. The economic loss is particularly great when using expensive powdery metal material. Further, in a case of compositing Al or Al alloy powder, or Cu or Cu alloy powder (hereinafter referred to as a matrix metal powder) with functional material of other metal or alloy powder, if the material is exposed to high temperature condition, particularly, semi-molten state, there is a problem that an intermetallic compound is formed at the boundary between both of them which greatly reduces the physical properties of the composite material molding product. Then, since cutting dusts containing the intermetallic compound cannot be utilized even if they are recovered as scraps, the economic loss is greater.
Referring next to the method (2) above, although it shows high material yield as compared with the method (1), it is necessary to apply extrusion and to cut the product into slabs prior to cold or hot forging, which results in wasteful cut portions, as well as requiring considerable cutting cost. In addition, shape control in the extrusion molding step is only possible for the two-dimensional control and three-dimension shape control is extremely difficult, thus imposing a restriction on the shape of the preliminary molding product as the object of the cold or hot forging. In addition to such problems, since starting powder material capable of near-net shaping has to be applied with extrusion molding into a rod-like slabs (material for forging) prior to the cold or hot forging, it is uneconomical in view of the material and the step to increase the production cost, unless special effect is recognized in the performance of the products.
On the contrary, the method (3) described above is improved as compared with the method (2) in view of the problems due to the formation of the extrusion molded slabs (resulting in many cut portion, difficulty in the shape of three-dimensional slab, near-net shaping, etc. Furthermore, mass production is possible by the introduction of a powdery forging facility for continuously practicing the respective steps of compacting, degreasing and forging, by which there can be made a considerable improvement in view of the economical merit. However, it is necessary in this method to blend a lubricant such as zinc stearate or wax upon mixing the starting powder material with an aim of improving the mold releasability upon extracting the compact molding product from a molding die.
Although the lubricant is decomposed and sublimated in the degreasing step, it partially remains in the compact molding product and causes reduction in the strength of the molding product. Also, the lubricant adheres to the surface of the compact molding product and deteriorates the surface properties after forging. In addition, since the degreasing step is usually conducted at a temperature higher than 450.degree. C., reaction is takes place between the matrix metal powder and the added functional metal powder to form an intermetallic compound at the boundary between them which deteriorates the physical properties. If the blending amount of the added metal powder is increased with an aim of compensating the reduction, there is another problem that other properties are deteriorated.